This invention relates to a method and a system for reducing the moisture content of a moist material and, more particularly, pertains to a drying system and method wherein the treating medium is drawn through the moist material to be treated.
Over the years various systems and methods have been developed for removing liquid components from moist materials such as sewage sludge, bagasse, de-inked paper sludge, biological sludge, wood pulp, bark, woodchips, slurry, peat and pasty products of different types. For instance, U.S. Pat. No. 4,768,292 issued to Manzei on Sep. 6, 1988 and U.S. Pat. No. 5,653,872 issued to Cohan on Aug. 5, 1997 each disclose a thermal drying system in which the sewage sludge to be treated is reduced into pellet masses and then exposed to a heated gaseous treating medium to drive off a portion of the moisture contents of the sewage sludge. The heated gaseous medium flows through the sewage sludge by operation of a blower fan.
More particularly, the thermal drying system described in U.S. Pat. No. 4,768,292 generally includes a pair of upper and lower porous conveyors vertically superposed and staggered so that the pellet masses will drop from the upper conveyor to the lower conveyor which is driven in a direction opposite to that of the upper conveyor. The hot gaseous medium travels successively and alternately from the layer of pellet masses through perforations defined in the supporting surfaces of the conveyors and from the perforations through the layer of pellet masses to provide an homogeneous and decreasing level of moisture.
Although the above thermal drying systems are effective for removing a portion of the moisture content of the moist material, they require a high level of energy as they all necessitate that the gaseous treating medium be heated at relatively high temperatures.
Accordingly, there is a need for a drying method which is economical while maintaining a high efficiency for removing liquids and/or moisture from the moist material to be treated. Another disadvantage of such thermal drying systems is that they dry an exterior layer of the pellets before the core thereof, thereby forming a barrier to the subsequent extraction of the moisture from the core of the pellets.
In another art, as illustrated by U.S. Pat. No. 4,219,942 issued to Coliva on Sep. 2, 1980, it is known to draw heated air through a damp fabric to dry the same and thus generate a shrinking and stabilizing effect. However, this drying system also requires a lot of energy as the drying process is still carried out by a thermal effect due to a flow of hot air.
Therefore, it would be highly valuable to introduce a drying system which could operate at ambient and low temperatures.
It is therefore an aim of the present invention to provide a drying system and method which requires a relatively low level of energy.
It is also an aim of the present invention to provide a drying system which could operate at ambient and relatively low temperatures.
It is also an aim of the present invention to provide a drying system in which the material to be treated is dried from a central portion to an outer layer thereof.
It is also an aim of the present invention to provide a drying system in which different residual moisture contents of the treated material may be obtained.
It is still an aim of the present invention to provide such a drying system which is adapted to minimize the time required to dry the moist material.
It is a further aim of the present invention to provide a method suitable for recycling waste materials into useful products such as fertilizers, soil conditioners, absorbent materials or combustion agents.
Therefore, in accordance with the present invention, there is provided a method for reducing moisture content of a moist material having an external surface, comprising the steps of increasing the exterior surface of the moist material, and drawing a relatively dry gaseous fluid through the moist material.
Also in accordance with the present invention, the method includes the step of separating the moist material into individual pieces prior to drawing a gaseous drying medium therethrough.
Typically, the gaseous drying medium flows by suction from an upstream side to a downstream side of each individual piece at a relatively high rate such as to sufficiently reduce the pressure on the downstream side of each individual piece and so causing moisture located in a central portion thereof to migrate near the external exposed surfaces of each individual piece substantially on the downstream side thereof.
Also in accordance with the present invention, there is provided a pelletizer for transforming a pasty material into a plurality of pellets. The pelletizer comprises a perforated plate means defining a plurality of openings, means for forcing the pasty material to pass through the perforated plate means from an inner surface thereof to an outer surface thereof, and cutting means adjacent the outer surface of the perforated plate means for transversally cutting the pasty material emanating from the perforated plate means.
Typically, the cutting means include a displaceable perforated plate means disposed downstream of the perforated plate means and defining a plurality of openings, and means for imparting a reciprocating motion to the displaceable plate means such as to alternately align and offset the openings of the displaceable perforated plate means with the openings of the perforated plate means.
Also in accordance with the present invention, there is provided a pelletizer for transforming a pasty material into a plurality of pellets, comprising a number of interchangeable perforated plate means, and means for forcing the pasty material to pass through one of the interchangeable perforated plate means selected according to the pasty material to be treated.
Also in accordance with the present invention, there is provided an apparatus for shredding solid materials into smaller pieces, comprising a housing defining a material inlet and a material outlet, at least two series of spaced- apart crowned discs mounted in an offset relationship on respective parallel shafts and rotatably driven in opposite directions for drawing and cutting the solid material supplied through the material inlet of the housing.
Also in accordance with the present invention, there is provided a screw conveyor comprising a substantially elongated casing defining opposed material inlet and outlet means and having an arcuate porous bottom wall, and a rotating screw axially mounted to the elongated casing for displacing a pasty material having a viscosity from the material inlet means to the material outlet means while at the same time compressing the pasty material against an inner surface of the arcuate porous bottom wall thereby causing some liquid from the pasty material to flow therethrough, and means for adjusting the inclination of the screw conveyor according to the viscosity of the pasty material to be treated.
Also in accordance with the present invention, there is provided a screw conveyor comprising a rotating screw mounted within an elongated casing for displacing a pasty material from a material inlet means to a material outlet means defined at opposed end portions of said elongated casing, the casing having a flexible arcuate bottom wall for allowing the screw conveyor to accommodate various quantity of pasty material.
Typically, the flexible arcuate bottom wall is releasably mounted to an upper portion of the elongated casing to permit disengagement thereof when the pressure exerted thereon reaches a critical point.